1. Field of the Invention
The present invention relates to a communication system, and more particularly, to a packet service system and method of controlling packet transmission.
2. Discussion of the Related Art
A universal mobile telecommunications system (UMTS) is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network 300 and wideband code division multiple access (W-CDMA) technology.
FIG. 1 shows a network structure of a general UMTS. As shown in FIG. 1, the UMTS is roughly divided into a terminal 100, a UTRAN 200 and a core network 300. The UTRAN 200 includes one or more radio network sub-systems (RNS). Each RNS includes a radio network control (RNC) and one or more Nodes B managed by the RNCs.
Nodes B are managed by the RNCs, receive information sent by the physical layer of a terminal 100 (e.g., mobile station, user equipment and/or subscriber unit) through an uplink, and transmit data to a terminal 100 through a downlink. Nodes B, thus, operate as access points of the UTRAN 200 for terminal 100.
The RNCs perform functions which include assigning and managing radio resources, and operate as an access point with respect to the core network 300. A primary function of UTRAN 200 is constructing and maintaining a radio access bearer (RAB) for a call connection between the terminal 100 and the core network 300. The core network 300 applies quality of service (QoS) requirements of end-to-end to the RAB, and accordingly, UTRAN 200 can satisfy the QoS requirements of the end-to-end by constructing and maintaining the RAB.
The RAB service is divided into an lu bearer service and a radio bearer service. The lu bearer service handles reliable user data transmission between boundary nodes of UTRAN 200 and the core network 300, while the radio bearer service handles reliable user data transmission between the terminal 100 and UTRAN 200.
FIG. 2 illustrates a radio protocol between the terminal 100 and UTRAN 200 on the basis of the 3GPP wireless access network standards. With reference to FIG. 2, the radio protocol is vertically formed of a physical layer, a data link layer and a network layer, and is horizontally divided into a user plane for transmitting data information and a control plane for transmitting a control signal.
The user plane is a region to which traffic information of a user, such as voice or an IP packet, is transmitted. The control plane is a region to which control information, such as an interface of a network or maintenance and management of a call, is transmitted. In FIG. 2, protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model well known in a communication system.
The first layer (PHY) provides an information transfer service to the upper layer by using various radio transfer techniques. The first layer is connected to the MAC layer through a transport channel, and data is transferred between the MAC layer and the PHY layer through the transport channel. The MAC layer provides a re-allocation service of the MAC parameter for allocation and re-allocation of radio resources.
The MAC layer is connected to the radio link control (RLC) layer through a logical channel, and various logical channels are provided according to the kind of transmitted information. In general, when information of the control plane is transmitted, a control channel is used. When information of the user plane is transmitted, a traffic channel is used.
The MAC is classified into an MAC-b sublayer, an MAC-d sublayer and an MAC-c/sh sublayer according to types of managed transport channels. The MAC-b sublayer manages a BCH (Broadcast Channel) handling broadcast of system information, while the MAC-c/sh sublayer manages shared transport channel such as FACH (Forward Access Channel), DSCH (Downlink Shared Channel), or the like, shared with other terminal 100s. 
In UTRAN 200, the MAC-c/sh sublayer is positioned at a control RNC (CRNC) and manages channels shared by every terminal 100 in a cell, so that one MAC-c/sh sublayer exists in each cell. The MAC-d sublayer manages a DCH (Dedicated Channel), a dedicated transport channel for a specific terminal 100. Accordingly, the MAC-d sublayer is positioned at a serving RNC (SRNC) managing a corresponding terminal 100, and one MAC-d sublayer exists also at each terminal 100.
A radio link control (RLC) layer supports a reliable data transmission and may perform a function of segmentation and concatenation of an RLC service data unit (SDU) coming from a higher layer. The RLC SDU transferred from the higher layer is adjusted in its size according to a throughput capacity at the RLC layer, to which header information is added, and then transferred in a form of a PDU (Protocol Data Unit) to the MAC layer. The RLC layer includes an RLC buffer for storing the RLC SDU or the RLC PDU coming from the higher layer.
A broadcast/multicast control (BMC) layer performs functions of scheduling a cell broadcast message (CB) transferred from the core network 300 and broadcasting the CB to UEs positioned in a specific cell(s). At the side of UTRAN 200, the CB message transferred from the upper layer is combined with information, such as a message ID, a serial number or a coding scheme, and transferred in a form of BMC message to the RLC layer and to the MAC layer through a CTCH (Common Traffic Channel), a logical channel. In this case, the logical channel CTCH is mapped to a FACH (Forward Access Channel), a transport channel, and an S-CCPCH (Secondary Common Control Physical Channel), which is a physical channel.
Referring to FIG. 2, a packet data convergence protocol (PDCP) layer is an upper layer of the RLC layer, allowing data to be transmitted effectively on a radio interface with a relatively small bandwidth through a network protocol such as the IPv4 or the IPv6. For this purpose, the PDCP layer performs a function of reducing unnecessary control information, which is called a header compression, and in this respect, RFC2507 and RFC3095 (robust header compression: ROHC), a header compression technique defined by an Internet standardization group called an IETF (Internet Engineering Task Force), can be used. In these methods, because the only information requisite for the header part of a data is transmitted, control information is transmitted, so that an amount of data transmission can be reduced.
The RRC layer positioned in the lowest portion of the third layer (L3) is defined only in the control plane and controls the logical channels, the transport channels, and the physical channels in relation to the setup, the reconfiguration, and the release of the RBs. The RB signifies a service provided by the second layer for data transmission between the terminal 100 and UTRAN 200, and setting up the RB means processes of stipulating the characteristics of a protocol layer and a channel, which are required for providing a specific service, and setting the respective detailed parameters and operation methods.
In order to broadcast or multicast the multimedia data to a specific terminal 100 group wirelessly, a new service called a multimedia broadcast/multicast service (MBMS) has been proposed. The MBMS is a service for transmitting multimedia data such as audio, video or image data to plural terminal 100s by using a uni-directional point-to-multipoint bearer service. The MBMS is divided into a broadcast mode and a multicast mode. That is, the MBMS is divided into an MBMS broadcast service and an MBMS multicast service.
The MBMS broadcast mode is a service for transmitting multimedia data to every user in a broadcast area. The broadcast area means a broadcast service available area. One or more broadcast areas may exist in one PLMN. One or more broadcast services can be provided in one broadcast area. And one broadcast service can be provided to several broadcast areas.
The MBMS multicast mode is a service for transmitting multimedia data only to a specific user group existing in a multicast area. The multicast area means a multicast service available area. One or more multicast areas can exist in one PLMN, one or more multicast services can be provided in one multicast area, and one multicast service can be provided to several multicast areas.
In the multicast mode, a user is requested to join a multicast group to receive a specific multicast service. At this time, the multicast group refers to a user group that receives the specific multicast service, and joining refers to a behavior of being admitted to the multicast group intending for receiving the specific multicast service. An MBMS RAB, a radio access bearer (RAB) for the MBMS, is configured for guaranteeing a specific level of QoS.
The MBMS serves packet transmission having real-time characteristics. Therefore, the MBMS uses a real-time transport protocol (hereinafter abbreviated RTP), which provides end-to-end delivery services for data with real-time characteristics, such as interactive audio and video real-time packet transmission. A RTP control protocol (hereinafter abbreviated RTCP) playing a role of real-time packet transmission control using the packet loss information is used because RTP itself does not provide any mechanism to ensure timely delivery or provide other quality-of-service guarantees.
The RTP and RTCP were originally developed for wired networks (i.e. Internet) and end-to-end corresponds to one data source and one final destination. The data source transmits the RTP packets to the UE through the radio and wired section. Here, radio section means the air interface from UTRAN 200 to UE, and wired section means the non-air interface from UTRAN 200 to data source via CN. Then, the UE transmits the RTCP packets having the packet loss information to the data source for the control of next RTP packet transmission.
The data source receives the RTCP packet and determines the next RTP packet transmission rate, size or encoding scheme based on the amount of packet loss during the RTP packet transmission. The data source having no way of determining loss in the radio section, considers the packet loss as collision in the wired section.
The UMTS system, however, consists of radio and wired sections and, moreover, the amount of packet loss in radio section is greater than the amount of packet loss in the wired section. When the packet loss only occurs in radio section, the data source cannot accurately determine where the packet loss happens and therefore presume that the loss is result of a collision in the wired section.
The packet loss occurring in radio or wired sections affect the determination of next RTP packet transmission rate, size, or encoding scheme. In the related art, there is no way for the data source to determine the cause for packet loss because the UE receives the RTP packets and calculates the amount of packet loss and transmits the RTCP packet including that information.
Further, in MBMS, a plurality of UEs receive the RTP packet of a MBMS service and the RTCP packets are transmitted to the data source from the respective UEs. The RTCP packet transmission from the plurality of UE to the data source happens simultaneously occupying radio resources and results in a problem of determining the bandwidth required for RTP and RTCP packet transmission.
Specifically, since RTP and RTCP are protocols based on the wired section, the usage of RTP and RTCP in point-to-multipoint communications as MBMS without modification results in inefficient control of packet transmission and errors in assigning the proper bandwidth required for the packet transmission.